I consider graph tooltip as extremely useful feature. It has some "caveat" though:

When items matching pattern have no data during selected time period, they are not shown in the tooltip. I believe, user should be informed also in the tooltip, that respective item (matching the pattern) has no data, similarly as in the legend of classic graph.

Also the count of metrics in the tooltip could be displayed also in case that there are 20 or less of them, similarly as when their count exceeds configured limit.

These both improvements could help user to notice that something is missing in respective graph.